The most popular liquid crystal display exemplified herein is mainly comprised of a thin film transistor (TFT) array substrate, a color filter substrate and a liquid crystal layer sandwiched between the two substrates. In a conventional TFT array substrate, an amorphous silicon (a-Si) TFT or a low-temperature polysilicon (LTPS) TFT often serves as a switching device of each sub-pixel. According to recent researches, the oxide semiconductor TFT has greater mobility than the a-Si TFT, and the oxide semiconductor TFT has a larger area and a lower production cost than the LTPS TFT. Hence, the oxide semiconductor TFT has great potential for becoming the key element of the next-generation flat display panel.
In a conventional oxide semiconductor TFT, the oxide channel layer is easily influenced by moisture and oxygen in the atmosphere, and the electrical property of the oxide channel layer may drift with time. Accordingly, the electrical performance and the reliability of the oxide semiconductor TFT are affected. Moreover, in the conventional oxide semiconductor TFT, the threshold voltage (Vth) of the oxide semiconductor layer is shifted due to the irradiation of a short wavelength light (i.e. an ultraviolet light), a device characteristic thereof is probably unstable, for example, a drain induced barrier lowering (DIBL) effect is probably occurred to cause a current leakage phenomenon of the device, which may influence a display quality. Accordingly, manufacturers now focus on improving the stability of an oxide channel layer by mitigating the influence of the external environment and the threshold voltage shift in an oxide semiconductor TFT resulted from the irradiation of a short wavelength light.